The technical case for VDSL2

VDSL2 is tailor-made for IPTV, especially HD, both in terms of bit rate and quality of service. At distances of up to 1 Km, VDLS2 is designed to deliver 50 Mbps downstream, but in practice the range is 32 to 40 Mbps, given that real bit rates are typically 65pc to 80pc of the theoretical limit.

The consensus is that such bit rates are needed for future triple-play services delivering multi-channel HDTV. But VDSL2 performance falls off more rapidly with distance compared with ADSL2+. In fact, when the copper loop length exceeds 1.5 Km (about 1 mile), VDSL2 does no better than ADSL2+. This has made 1 Km the target maximum loop length for many VDSL2 deployments. In Europe only about 20pc of the population, on average, lies within this range, according to Alcatel’s Danny Goderis, so that substantial investment in more fibre is needed in the long-term. Because a much greater variety of applications has been envisioned than in the early days of DSL, VDSL2 introduced the concept of “profiles”, dividing the standard into a series of subsets each designed to match specific, sometimes conflicting, requirements. Using profiles, VDSL2 can be optimised for several distinct applications, one of which is IPTV. The key feature here is a mechanism called Virtual Noise, designed to minimize the impact of interference from nearby copper circuits while maximising the bit rate. Problems sometimes arise when a neighbour switches on a modem, generating interference that causes the modem to drop the link with the DSLAM and re-synchronise. This is not a problem for ordinary data, but it can interrupt video.

Existing DSL standards such as ADSL2+ set static noise buffers to cope with such interference, but these operate continuously whether or not there is a problem, with the result that potential bit rate is sacrificed needlessly. With VDSL2, the noise levels are set dynamically according to the condition of the circuit as measured at that instant, so that bit rates only drop temporarily in the event of interference. The result might be a transient deterioration in picture quality, but this is a price worth paying for better service the rest of the time. VDSL2 is also an attractive option for distributing IPTV within multi-tenanted buildings from fibre nodes in the basement, where loop lengths are typically under 100 metres. Additional reporting: Philip Hunter